Startup Conducts First Subsea Geothermal Power Plant Test Near Underwater Volcano

Seattle-based startup Endurance Energy reported the world's first demonstration of a subsea geothermal power plant near an active underwater volcano in the northeast Pacific Ocean. The three-week test deployment in September at the Axial Seamount generated less than 10 seconds of net-positive electricity, marking a preliminary technical test for tapping underwater renewable energy.

According to Endurance Energy founder and CEO Andrew Redd, the deep ocean provides unlimited cold seawater for cooling while water pressure keeps the system and well pressurized. However, independent experts note that seafloor geothermal operations face distinct engineering hurdles that could increase installation and operational costs compared to land-based facilities.

Test Deployment and Technical Challenges

During the September deployment, the team utilized a subsea lander drill deployed from the ship Ocean Guardian to drill two holes in the seafloor at a depth of approximately 5,000 feet. Both holes vented hydrothermal fluid measured at about 500° F. The company cased and cemented the second hole and installed a valve to create a well, then connected its 100-kilowatt power plant named Adélie using Canada's ROPOS submersible.

The initial test well extended 52 feet down into the seafloor and was 2 inches wide. Because it was kept small and shallow, it did not deliver enough hydrothermal fluid to run the power plant continuously. To complete a test run, the company temporarily redirected Adélie to a nearby natural hydrothermal vent flowing at about 538° F on September 27. Over nearly half a minute, the turbine reached about 26,000 rpm and produced 60 watt-hours of electricity, achieving net-positive power production for about 8.7 seconds with a peak of 16.5 kilowatts.

The test run concluded following a software safety stop triggered by a battery module reporting a current spike, followed by a short circuit in a refrigerant pump cable caused by seawater seepage.

Future Plans and Commercial Outlook

Endurance Energy plans to conduct a winter 2026–2027 trial using a refurbished and upgraded version of the power plant named Gentoo. The company is also developing a larger 2-megawatt commercial generator for a planned commercial deployment with the Kingdom of Tonga in 2027, where electricity is primarily generated from imported diesel fuel.

Experts such as Stanford University energy science professor Roland Horne have cautioned that while the approach is technically feasible, seafloor geothermal systems must prove cost-competitive against land-based alternatives and demonstrate long-term operational reliability.

Practical Implications and Verification

• Technology Status: The project remains in the prototype and demonstration phase following initial short-duration tests.
• Environmental Monitoring: The startup reported conducting water sampling and monitoring seafloor ecosystems during the deployment.
• Verification: Interested observers can review independent scientific literature regarding the Regional Cabled Array and announcements from Endurance Energy and participating research institutions.